The OFDM modulation format is used in many communication standards. The OFDM technique is used in particular for mobile communication, since it represents an effective measure against selective signal fading.
In OFDM systems, the available frequency spectrum is subdivided into a large number of narrowband orthogonal subcarriers. An OFDM symbol is transmitted via a plurality of these subcarriers (possibly all of these subcarriers). In order to demodulate the received OFDM symbols in the receiver, the receiver continuously estimates the channel transfer function (CTF). The CTF is estimated using pilot information which is embedded in the time/frequency lattice covered by the OFDM symbol stream.
Conventional channel estimators measure the CTF associated with an OFDM symbol using the pilot information transmitted via the channel, and equalize the OFDM symbol using the determined CTF. Furthermore, it is already known for the determined CTF to be transformed to the time domain, for the channel impulse response (CIR) obtained by the transformation to be processed in such a way that signal components occurring after more than a maximum delay time are removed from the CIR, and for the CIR which has been “cleaned” in this way to be transformed back to the frequency domain.
A channel estimate should produce the least possible mean square error (MSE) over a multiplicity of possible channel scenarios (different delay profiles, Doppler shifts and signal-to-noise ratios). It has been possible to show that the optimum channel estimator for OFDM systems is a Wiener filter. However, Wiener filters involve a high degree of computation complexity and, in consequence, a high level of receiver complexity. Sub-optimum channel estimators allow the complexity to be reduced at the expense of loss of performance in the channel estimation process. In practice, the channel estimate is therefore a compromise between MSE performance, receiver complexity, latency, robustness and, possibly, further influencing variables.
For these and other reasons, there is a need to develop new algorithms and apparatuses for channel estimation in OFDM systems.